1. Field of the Invention
The present invention relates to a manufacturing apparatus that manufactures a hub unit and that is used in a process of clinching a shaft end of an inner shaft after an inner ring member is press-fitted over the shaft end of the inner shaft. In particular, the present invention relates to a manufacturing apparatus in which a main spindle supporting a clinching punch has an improved strength.
2. Description of Related Art
A hub unit is used as a bearing apparatus that supports wheels of a vehicle such that the wheels are rotatable. In an inner shaft of the hub unit, an inner ring member is assembled into a shaft end of a hub shaft. During a process of manufacturing a hub unit, the shaft end of the hub shaft is clinched so that the assembled inner ring member is prevented from slipping out from the shaft end. For example, an oscillating clinching apparatus in Japanese Patent Application Publication No. 2000-38005 (JP 2000-38005 A) is used for this clinching process.
FIG. 8 schematically illustrates a clinching method using an oscillating clinching apparatus 100. A hub shaft 101 has a cylindrical shaft end portion 106 at a shaft end thereof. An inner ring member 102 is press-fitted over the shaft end portion 106. In this case, a part of the shaft end portion 106 protrudes from an end surface of an inner ring member 102. In the oscillating clinching apparatus 100, a punch 104 is pressed against the protruding part to push the protruding part outward in a radial direction. In FIG. 8, the shape of the shaft end portion 106 that has not been plastically deformed is depicted by a long dashed double-short dashed line.
The punch 104 is installed at a distal end of a main spindle 105. The main spindle 105 is inclined at a predetermined angle θ to an axis of the oscillating clinching apparatus. Thus, the punch 104 contacts the shaft end portion 106 at one point on a circumference of the shaft end portion 106. Pressing the punch 104 against the shaft end portion 106 causes the contact portion to be plastically deformed outward in the radial direction. When the main spindle 105 is lowered toward the hub shaft while being pivoted around the axis of the hub shaft 101, the shaft end portion 106 is plastically deformed outward in the radial direction all over a circumference thereof. In this manner, the oscillating clinching apparatus 100 partly plastically deforms the shaft end portion 106 to allow the shaft end portion 106 to be machined with a relatively low clinching load compared to a method of simultaneously plastically deforming the entire circumference of the shaft end portion 106.
Thus, a clinching portion 103 is formed at the shaft end of the hub shaft 101. The clinching portion 103 constrains movement of the inner ring member 102 in an axial direction to prevent the inner ring member 102 from slipping out.
The punch 104 is changed to a different punch as needed according to the size of the hub unit 107. Thus, a punch holder 108 to which the punch 104 is attached is formed at the spindle end of the main spindle 105. The punch holder 108 has outside dimensions equivalent to outside dimensions of the punch 104, and has a relatively large diameter. On the other hand, a spindle portion 109 corresponding to the whole main spindle 105 except for the punch holder 108 supports only a relatively low clinching load, and needs only a small diameter dimension. An example of the shape of the spindle portion 109 with a reduced diameter dimension is depicted in FIG. 8 by a dashed line. Reducing the diameter dimension of the spindle portion 109 allows the oscillating clinching apparatus 100 to be made compact. As described above, in the main spindle 105 of the oscillating clinching apparatus 100, the spindle portion 109 is shaped like a cylinder with a small diameter and provided with the punch holder 108 with a large diameter at the spindle end of the spindle portion 109.
However, the above-described shape involves a “joint portion” K formed at a portion of the main spindle 105 where the punch holder 108 and the spindle portion 109 are joined together. When the oscillating clinching apparatus 100 with the main spindle 105 assembled therein is used as a mass production facility to manufacture a large number of hub units, the “joint portion” K may disadvantageously be broken relatively prematurely.